1. Field of the Invention
The present invention relates to a process for producing semiconductor device.
2. Background Information
Conventionally, when an SOI (silicon-on-insulator) device formed on an insulator is combined or united with a device formed on a silicon substrate, such as a power MOS transistor, etc., the resultant increase in current capacity causes the substrate to act as a drain, and thereby, the device formed on the insulator occasionally becomes unstable due to a fluctuation of the drain voltage occurring upon operation of the power MOS transistor.
A solution to this problem was proposed in Japanese Unexamined Patent Publication (Kokai) No. 62-213272. In the proposed arrangement as shown in FIG. 1, on a LOCOS-oxidized film as a first insulating layer 201, an electroconductive thin film 204 as a shield is formed by the same process steps as the forming of the gate electrode 203 of a vertical MOS transistor 202, a second insulating layer 205 is formed on the electroconductive thin film 204, and semiconductor elements 206 and 207 are then formed on the second insulating layer 205 so that the thus formed SOI device is shielded from a fluctuation of the substrate electric potential due to the operation of a vertical MOS transistor.
In this arrangement, however, the SOI device portion naturally has a greater height than the vertical MOS transistor region as seen from FIG. 1, and the thus formed uneven or stepped surface causes a disconnection of conductor layers at the steps, particularly in highly integrated devices having a multilayered conductor structure.
Moreover, such an arrangement is not advantageous as an IC circuit in that an SOI device formed by depositing a polycrystalline silicon (hereinafter simply referred to as "polysilicon") on the second insulating layer cannot provide a high device performance obtained when an SOI device is formed in a monocrystalline silicon.
The present inventor intended to make even or flat surfaces on the SOI device portion and the vertical element portion. This arrangement, however, requires that an SOI device must be insulation-separated and shielded not only in the downward direction but also in the sideward direction, which causes a problem of increased and complicated process steps for the sideward insulation separation and shielding, including forming of a trench hole from the surface, an insulating layer, a shield layer, and further, a connection between a lower shield and side shield layers.